This invention relates generally to the art of levelling, and more particularly, to a spirit level having illumination means for measuring the level of a surface in dim light or darkness.
The art of providing level measuring devices having illumination means has been pursued for many years. Indeed, various levels have been developed wherein lamps are used to illuminate the area around the bubble tubes. A system of conductor wires has been used to convey current from a battery source to the lamps. In the past, this method of conductivity has been unsuccessful because the components have been unable to endure vibration from handling and from general use. Without a means for dampening vibration, the life of such an electrical system is shortened.
Hansen in U.S. Pat. No. 3,584,212 teaches a spirit level containing a number of bubble tubes illuminated by small lamps connected to a battery source by a system of wires. A similar device is disclosed in U.S. Pat. No. 2,806,133 to Aube wherein a level includes a number of lamps, conductor wires, and a battery source comprising a means to illuminate the bubble tubes. Similarly, U.S. Pat. No. 4,484,393 to LaFreniere describes a level having a bubble tube which is illuminated by two light paths directed through it.
Attempts to discourage the damage caused by vibration have been pursued typically by placing a lamp, or lamps, in direct contact with a battery source. This eliminates the need for conductor wires. However, such a method limits the number of lamps that can be used. Hubbard in U.S. Pat. No. 3,003,054 teaches a spirit level illuminated by a single lamp directly connected to a battery source to eliminate the need for conductor wires which may be damaged by vibration. Another such device is disclosed in U.S. Pat. No. 4,745,687 to Wilhelmy wherein an illuminated level for a recreational vehicle comprises a liquid-filled container which is illuminated by a single lamp connected to a battery. U.S. Pat. No. 4,833,788 to Munro discloses an attachable apparatus of two lamps and a battery source which can be externally connected to a level.
The prior art has failed, however, to provide an illuminated spirit level which is adequately durable to resist breakage in use.